The present invention relates to absorption spectroscopy methods and apparatus, and in particular to those methods and apparatus which employ an optical cavity for increasing detection sensitivity, especially ones adapted for cavity ring-down spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS). Arrangements, active or passive, for reducing sensitivity of the instrument to alignment or vibrations, including those that manipulate or control optical resonances of the instrument""s cavity are particularly relevant.
Cavity ringdown spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS) methods and associated instruments employ optical cavities (also known as xe2x80x9cetalonsxe2x80x9d) as absorption cells for spectroscopic purposes. These spectroscopy methods and instruments have a broad range of other applications, such as characterizing mirror reflectivities, determining optical cavity losses (including scattering, absorption, etc.) and measuring thin film absorption. Other potential uses include using the invention for quantitative chemical analysis systems for applications such as offgas monitoring, medical diagnostics (such as breath analysis), trace gas analysis, thin film analysis, pollution monitoring, process control monitoring, purity analysis, and toxic chemicals detection. Although the background discussion will focus on the specific development of absorption techniques for gas phase chemical detection and characterizing optical components such as high reflectivity, low loss mirrors, it is by no means limited to this application.
The state of the art in spectrophotometer technology and spectroscopic techniques used for the purpose of spectrally characterizing solids, liquids, and gases includes absorption, emission, and ionization-based techniques. Recent developments in optical components and spectrally bright light sources have led to a variety of spectroscopic-based techniques and instruments that provide chemical analysis data of gaseous samples. Conventional Absorption (CA) or Emission (EM) spectroscopies are currently implemented for the quantitative analysis of chemical species in gases as a means of providing concentration information, but these methods frequently require tedious calibration procedures (EM) or suffer from low sensitivity. In the case of emission spectroscopy, inter and intramolecular dynamics, such as internal conversion or predissociation, can significantly degrade the ability to both detect and quantify species concentrations. Although much less effected by these dynamical processes, CA historically suffers from lower sensitivity (compared to that of EM, for example), and hence cannot typically achieve similar detection sensitivities. Another technology currently in use for chemical monitoring is Laser Spark Spectroscopy (LS), which involves measuring emission spectra of species that are vaporized with an intense laser pulse. Although highly sensitive, LS is only capable of identifying the presence of metals, and is not generally capable of providing absolute concentrations for those species without elaborate calibration procedures. A primary reason for employing CA is the relative ease with which absolute species concentration can be determined from the associated absorption spectra. To circumvent the historically lower sensitivity of absorption spectroscopy, several methods have been developed. Absorption-based optical detection methods which enable chemical concentrations to be determined include frequency (or amplitude) modulated laser absorption spectroscopy (FM-LAS) and Fourier transform spectroscopy (FTS). Although these methods have been used with some success, they can suffer from low sensitivity (FTS), or can be difficult or impossible to implement in spectrally congested areas and have limited spectral coverage (FM-LAS).
An alternative to these methods involves the use of high finesse optical cavities, which have long been known to amplify optical loss processes occurring between the cavity optics. (Jackson, D. A., The Spherical Fabry-Perot Interferometer as an Instrument of High Resolving Power for use with External or with Internal Atomic Beams. Proc. R. Soc. London Ser. A, 1961. 263: p. 289.) Ultimately, this allows highly sensitive measurements of such processes as molecular absorption to be achieved. Several methods have been described to use optical cavities for such purposes. (Scherer, J. J., et al., Cavity ringdown laser absorption spectroscopyxe2x80x94history, development, and application to pulsed molecular beams. Chemical Reviews, 1997 January-February. 97(1): p. 25-51. Romanini, D., A. A. Kachanov, and F. Stoeckel, Diode laser cavity ring down spectroscopy. Chemical Physics Letters, 30 May 1997. 270(5-6): p. 538-45. Ye, J., L.-S. Ma, and J. L. Hall, Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy. J. Opt. Soc. Am. B, 1998. 15(1): p. 6. Paldus, B. A., et al., Cavity-locked ring-down spectroscopy. Journal of Applied Physics, 15 Apr. 1998. 83(8): p. 3991-7.) One of the most common, known as cavity ringdown spectroscopy (CRDS), measures the total optical cavity loss by monitoring the decay of the intracavity intensity following the injection of radiation into the cavity, either by a single laser pulse, (O""Keefe, A. and D. A. G. Deacon, Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources. Review of Scientific Instruments, December 1988. 59(12): p. 2544-51.) or by an abruptly interrupted continuous-wave (CW) laser. (Anderson, D. Z., J. C. Frisch, and C. S. Masser, Optical reflectometer based on optical decay time. Appl. Opt., 1984. 23: p. 1238.) The ringdown process itself was patented by Litton Corporation (U.S. Pat. No. 4,793,709, Method and apparatus for measuring losses of an optical cavity, issued Dec. 27, 1988, now expired.) for the specific purpose of determining mirror reflectivities. Since this patent, CRDS has been developed (and extensively published in the open literature) for the specific purpose of determining atomic and molecular absorption for species located within the optical cavity (between the mirrors). The currently well established and practiced pulsed CRDS method was first developed by O""Keefe and Deacon in 1988, who demonstrated its high sensitivity and spectroscopic capabilities by measuring weak visible absorption by molecular oxygen. (O""Keefe, A. and D. A. G. Deacon, Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources. Review of Scientific Instruments, December 1988. 59(12): p. 2544-51.) Additionally, continuous-wave versions of CRDS that are based on the original, early versions of the technology have been developed for the specific task of obtaining absorption spectra of chemical species placed in the cavity. (U.S. Pat. No. 5,528,040, Ring-down cavity spectroscopy cell using continuous wave excitation for trace species detection, Jun. 18, 1996.) Other cavity-based methods, such as integrated cavity output spectroscopy (ICOS) (O""Keefe, A., J. J. Scherer, and J. B. Paul, CW integrated cavity output spectroscopy. Chemical Physics Letters, 9 Jul. 1999. 307(5-6): p. 343-9.) and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) (Ye, J., L.-S. Ma, and J. L. Hall, Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy. J. Opt. Soc. Am. B, 1998. 15(1): p. 6.) use the cavity transmission properties to gauge the intracavity loss, but in these cases the intrinsic cavity loss must be determined separately, by using CRDS for example, to obtain quantitative absorption intensity data.
Many of the above methods, in particular those employing narrowband CW laser sources, are designed to manipulate or control the optical resonances that arise within cavity due to the periodic boundary conditions imposed on the intracavity electric field by the mirror surfaces. These resonances, which are interferometric in nature, comprise the general subject of Fabry-Perot theory. To control them precisely requires complex and expensive instrumentation and hardware, and places extreme constraints on the overall stability of the apparatus.
One solution to the problem of mode-buildup within the cavity involved intentionally creating a dense transverse cavity mode spectrum by employing particular cavity lengths that ensured a frequency spread among the various transverse modes, thereby filling in the region of the spectrum falling between the longitudinal cavity modes. (Meijer, G., et al., Coherent cavity ring down spectrometry. Chemical Physics Letters, 7 Jan. 1994. 217(1,2): p.112-6.) While the laser was aligned on-axis with the cavity, it was also slightly divergent upon entering the cavity to ensure transverse mode overlap. While similar, we believe this approach to be inferior to that presented below primarily for two reasons. Firstly, with their approach, light at different frequencies takes different paths through the cavity, while with the approach described below all of the light takes substantially the same path. Different light paths are subject to different levels of optical intensity loss due to such factors as variations in the mirror surface and diffraction at the edges of the mirrors. Therefore, our approach should provide an inherently smoother frequency response from the cavity. Secondly, we believe our off-axis approach is more capable of creating the densest possible cavity mode spectrum. This aspect is critical to the success of the method, as the goal is to create a virtual continuum of modes in order to completely flatten the optical frequency response of the cavity.
The solution represented by the present invention utilizes off-axis paths through spherical or astigmatic mirror interferometers to systematically disrupt these resonances, rather than attempting to manipulate them in other ways. The goal is to remove the frequency selectivity of the cavity, or to effectively decrease the cavity Longitudinal mode spacing such that a nearly continuous cavity transmission spectrum results, rendering it effectively a broadband device. This approach offers the following advantages over previous methods. Most importantly, narrowband lasers (xcex94v less than 100 MHz) can be used without actively controlling the cavity length. This eliminates the need for expensive components such as piezo-electric transducers, lock-in amplifiers, acousto-optic modulators, etc. Additionally, this design reduces the optical feedback from cavity to the source laser, which is particularly important for the case of a distributed-feedback diode laser sources. Previously, either Faraday isolators, which are expensive and not available over much of the infrared spectrum, or three-mirror ring-cavities (U.S. Pat. No. 5,912,740, Ring-resonant cavities for spectroscopy, Jun. 15, 1999.) have been used for this purpose. Finally, the constraints on the overall system alignment are vastly reduced. Rather than only a single possible alignment geometry (i.e. the laser on-axis with the cavity), any stable path (see below) through the cavity can be used. This allows simpler alignment routines, and lowers the sensitivity of the instrument to vibration. Off-axis cavity ringdown spectroscopy (OA-CR) and off-axis integrated cavity output spectroscopy (OA-I) methods and associated instruments allow high finesse optical cavities (also known as xe2x80x9cetalonsxe2x80x9d) to be used as absorption cells for spectroscopic purposes with significantly reduced complexity compared with previous approaches.